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Experimental data and theoretical calculations have failed to rationalize the role of covalnecy in real experimental BTP (2,6-Di(1,2,4-trianzin-3-yl)pyridine) complexes. DFT calculations on [M(BTP).sub.3].super.3} (M-La, U, Cm, Gd) were used to show that even if back-bonding effects are significant in the U-BTP bond, it is the contrast of donation on 6d and 5f [Cm.super.III] orbitals that explains, at least in part, its selective complexation to BTP.

The stability of a peroxo-iron species in the second time scale when reacted with H2O2 is due to the E47A mutation in superoxide reductase (SOR) from Desulfoarculus baarsii. Further characterization on the basis of Mossbauer and spin Hamiltonian parameters shows that the peroxo-Fe(super +3) species is a high-spin side-on complex.

Research has been conducted on the triosodide octanocoordinated uranium(III) complexes and their lanthanum(III) analogues. The crystal structures of these complexes has been investigated and the details are reported.

Research was conducted to examine various spectroscopic studies of WOs and MoRu porphyrin dimers using variable-temperature 1H NMR, resonance Raman, magnetic susceptibility and X-ray crystallography. The objective was to determine a proper molecular orbital diagram for the dimers. Results demonstrate an unusual molecular orbital diagram which has never been observed before in metal-metal bonded metalloporphyrin dimers.

A theoretical study was conducted to analyze bonding in lanthanide trihalides using density functional techniques. The local spin density approximation accommodated exchange and correlation potentials. Geometries were then optimized in the pyramidal and planar conformations by an energy-only minimizer. Results indicated the effectiveness of density functional computations to duplicate both structural and thermodynamic experimental information on the compounds.

Research was conducted to examine the magnetic properties of several mixed-valent diruthenium long-chain carboxylates with the general formula Ru2(RCO2)4X in temperatures ranging from 6-400 K. Magnetic susceptibility calculations were performed on the carboxylates. The columnar mesophase of the mesomorphic congeners suggests that there is no substantial structural change that takes place at the crystal-liquid crystal transition.

Chemical research has established the solid state magnetic susceptibility properties of group eight metal-to-metal doubly bonded porphyrin and tetraazaporphyrin dimers [Ru(Octaethylporphyrin)]2, Ru(octaethyltetraazaporphyrin)]2 and [Os(OEP)]. Nuclear magnetic resonance analysis indicates that each of the group eight dimers exhibits similar behaviour in zero-field splitting of the triplet ground electronic state.

The structural characterization of stable complexes of pentavalent uranyl synthesized from the reaction of the complex with the bulky amine-phenolate ligand potassium salt [K.sub.2](salan-[super]t[Bu.sub.2]) or the Schiff base ligand potassium salt [K.sub.2](salophen-[super]t[Bu.sun.2]) in pyridine is described. The presence of a back-bonding interaction in the pentavalent uranyl complexes possibly contributed significantly to the observed solution stability of the [U[O.sub.2](salan-[super]t[Bu.sub.2])(py)K] and [U[O.sub.2](salophen-[super]t[Bu.sub.2])(py)K] complexes with respect to disproportionation and hydrolysis.

The f-f transitions of a series of rare earth aquo and chloro complexes were theoretically evaluated to find out the experimental uncertainties concerning the coordination mode of trivalent plutonium in concentrated LiCl. The nephelauxetic effect from aquo to chloro complexes is clearly observed through both spectral red shifts and the decrease in [Fe.sub.2], the Slater-Condon parameter.

A study demonstrates how topological and more classical orbital analyses provide an overall coherent picture of metal-ligand bonding in heavy-metal complexes. Low magnitude covalent interactions, such as back-donation in actinide molecular (Am) complexes, are described with the help of topological approaches.

The first structural and magnetic investigations of a polar, unsupported Mo-Ru bond were conducted. When combined with magnetic susceptibility data, the crystal structure of ((OEP)MoRu(TPP))PF6, 1+PF6- (OEP = octaethylporphyrin, TPP = tetraphenylporphyrin) offers a wide range of data regarding the molecular orbital diagram for inter-triad heterometallic multiple bonds.

A theoretical and experimental study was conducted to determine the electronic, magnetic, and spectroscopic properties of mixed valent and divalent binuclear diruthenium tetracarboxylates. Using ZINDO/S-MRCI and DFT calculations, it was found that both divalent species have ground-state configuration and have robust Zero Field Splitting. Findings also indicate that a balance between the Ru-Ru and Ru-O (equatorial carboxylate) bond strengths is the major factor that determines their structural and spectroscopic properties.

The design of bistable magnetic systems should enable the storage of information by manipulation of the spin degrees of freedom. However, such a strategy relies on the preparation of target objects, whose environment must be controlled to favor a hysteretic behavior. Here, we report the successful modeling of a highly cooperative two-step spin-crossover iron(II) compound, [Fe(bapbpy)(NCS)2]. The magnetic susceptibility measurements and low- and high-temperature hysteretic cycles reflect the presence of an intermediate phase, which controls the memory-storage capacity of this material. It is shown that the hysteresis loop widths can be traced theoretically by evaluating the electrostatic contributions between the transiting units. Despite the apparent reduction of intermolecular interactions upon cooling, it is suggested that the enhanced fluctuations of the Madelung field are responsible for the observed hysteresis width changes. This counterintuitive scenario makes the preparation of information storage devices an even more challenging task, where theoretical inspections are very insightful. [ABSTRACT FROM AUTHOR]

The As III binding of two NTA-based tripodal pseudopeptides, possessing three cysteine (ligand L 1 ) or d-penicillamine residues (ligand L 2 ) as potential coordinating groups for soft semimetals or metal ions, was studied by experimental (UV, CD, NMR, and ESI-MS) and theoretical (DFT) methods. All of the experimental data, obtained with the variation of the As III :ligand concentration ratios or pH values in some instances, evidence the exclusive formation of species with an AsS 3 -type coordination mode. The UV-monitored titration of the ligands with arsenous acid at pH = 7.0 provided an absorbance data set that allowed for the determination of apparent stability constants of the forming species. The obtained stabilities (log K ' = 5.26 (As L 1 ) and log K ' = 3.04 (As L 2 )) reflect high affinities, especially for the sterically less restricted cysteine derivative. DFT calculated structures correlate well with the spectroscopic results and, in line with the 1 H NMR data, indicate a preference for the all- endo conformers resembling the As III environment at the semimetal binding sites in various metalloproteins.

Nickel superoxide dismutase (NiSOD) is an enzyme that protects cells against O 2 · - . While the structure of its active site is known, the mechanism of the catalytic cycle is still not elucidated. Its active site displays a square planar Ni II center with two thiolates, the terminal amine and an amidate. We report here a bioinspired Ni II complex built on an ATCUN-like binding motif modulated with one cysteine, which demonstrates catalytic SOD activity in water ( k cat = 8.4(2) × 10 5 M -1 s -1 at pH = 8.1). Its reactivity with O 2 · - was also studied in acetonitrile allowing trapping two different short-lived species that were characterized by electron paramagnetic resonance or spectroelectrochemistry and a combination of density functional theory (DFT) and time-dependent DFT calculations. Based on these observations, we propose that O 2 · - interacts first with the complex outer sphere through a H-bond with the peptide scaffold in a [Ni II O 2 · - ] species. This first species could then evolve into a Ni III hydroperoxo inner sphere species through a reaction driven by protonation that is thermodynamically highly favored according to DFT calculations.

A mononuclear nonheme iron(V)-imido complex bearing a tetraamido macrocyclic ligand (TAML), [Fe V (NTs)(TAML)] - (1), was oxidized by one-electron oxidants, affording formation of an iron(V)-imido TAML cation radical species, [Fe V (NTs)(TAML +• )] (2); 2 is a diamagnetic (S = 0) complex, resulting from the antiferromagnetic coupling of the low-spin iron(V) ion (S = 1/2) with the one-electron oxidized ligand (TAML +• ). 2 is a competent oxidant in C-H bond functionalization and nitrene transfer reaction, showing that the reactivity of 2 is greater than that of 1.

The low-energy spectroscopies of Mn(II) and Mn(III) porphyrin (P) complexes were investigated using complete active space and subsequent perturbative treatment (CASPT2) as well as DFT-based calculations. Starting from DFT optimizations of Mn(II)P and Mn(III)PCl using crystallographic data, the CASPT2 results show that whatever the relative position of the Mn(II) ion with respect to the porphyrin cavity, the high-spin state S = 5/2 of the [MnP] unit lies much lower in energy than the intermediate S = 3/2 state. Not only are these results in agreement with experimental observations but they also differ from previous theoretical conclusions. In the Mn(III) complexes, σ and π charge redistributions compete to result in a S = 2 ground state. The performances of different functionals have been tested in the reproduction of the CASPT2 spin gaps. Our results confirm that the Mn(II) system is very challenging, as GGA functionals fail in the spin states ordering and in the reproduction of the gaps, unless a high percentage of exact HF exchange (55%), as in KMLYP, is incorporated. This inspection demonstrates the need for specific active space functional to investigate the low-energy spectroscopy of [MnP] units.

The calculation of the absorption spectra of four families of transition-metal complexes (Ni(CO)4, MnO4(-), MF6 (M = Cr, Mo, W) and CpM(CO)2 (M = Rh, Ir)) has been undertaken to unravel the influence of basis sets onto excitation energies, oscillator strengths, and assignments. Three among the most common pseudopotentials, with the corresponding valence basis sets, and two all-electron basis sets have been used for the metal center description in the framework of the time dependent Density Functional Theory (TD-DFT). Our results show that this approach does not particularly depend on the basis set used on the metal atoms. Furthermore, the chosen functional PBE0 provides transitions in good agreement with experiments, and it provides an accuracy of about 0.3 eV, comparable to that of refined post-Hartree-Fock methods.

Solution (VT NMR, Evans method magnetic susceptibility, resonance Raman) and solid-state (SQUID magnetic susceptibility, X-ray crystallography) spectroscopic studies of intertriad heterodimeric [(OEP)MoRu(OEP)] (1), [(OEP)WOs(OEP)] (2), and [(OEP)MoRu(TPP)]PF(6) (3(+)) metalloporphyrins are reported (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinato; TPP = 5,10,15,20-tetraphenylporphyrinato). Solution and solid-state magnetic susceptibility data indicate that 1 and 2 contain two unpaired electrons in the ground electronic configuration. The presence of a delta bond in 3(+) has been confirmed by structural characterization. The experimental evidence is consistent with a molecular orbital ordering, sigma < pi < delta < pi < delta, which is different from that seen for homologous metalloporphyrin dimers with homometallic or intratriad heterometallic multiple metal-metal bonds. Resonance Raman data suggest that the heterometallic bonds are slightly stronger than isoelectronic homometallic species.